Power plane-to-power plane shorts are a common defect that typically necessitate scrapping an electrical component if they are not detected and repaired during the early stages of manufacturing. Locating shorts poses several significant yield and diagnostic problems. Firstly, shorts between power planes in an electronic package such as an MCM-D (multi-chip module provided with thin film layers) or a PCB (printed circuit board) are a generic and perennial problem both for yield and diagnostic considerations. Typically, an ohmmeter is used to determine the presence of power plane-to-power plane defects. However, to precisely locate the short is usually a formidable task, since such a defect can appear anywhere in the active area between adjoining power planes. Additionally, the complex nature of thin-film packaging makes it prohibitively expensive to inspect the entire MCM-D or PCB in order to determine the precise location of a short with a reasonable degree of accuracy. It is, therefore, highly desirable to have a method to pinpoint the location of power plane-to-power plane shorts in the most effective manner possible.
A second problem involves reliability considerations. Certain defects may be exceedingly tenuous, such as filamentary shorts which, typically, blow out when traversed by an even minimal current. For diagnostic considerations, it would be desirable to have a method for locating these fine shorts without destroying them, to gain valuable diagnostic information and helpful hints for an improved manufacturing process. Attempting to view such shorts with infrared techniques most likely will blow the short. Accordingly, as in the previous case, it is highly desirable to subject the short to minimal current stress while attempting to locate the short.
A third problem occurs in some shorts having a high resistance, and oftentimes, a significant amount of power plane-to-power plane capacitance. A method which can locate high resistance shorts in the presence of high power plane-to-power plane capacitance would therefore be highly beneficial.
A fourth problem, involves a specific defect known to be fatal: I/O net-to-power plane shorts. These defects oftentimes occur in an MCM-D on the highly dense TSM (Top Side Metallurgy, i.e., the side of the substrate to which chips are attached) that also involves a BSM (Bottom Side Metallurgy, i.e., the surface of the substrate to which pins are attached I/O nets). Repair opportunities for such nets are typically limited. In the presence of many levels of thin-films, shorts in this category frequently involve a power plane (rather than the more complex case of shorts between I/O nets). Moreover, both, electrical test and automated optical inspection techniques tend to be expensive. Thus, a simple, cost effective method of detecting and locating critical I/O net-to-power plane shorts is likewise highly desirable.
In summary, there is a need in the art of manufacturing MCMs and PCBs for an inspection technique which provides a sensitive and non-invasive approach to locating plane-to-plane defects. These include power plane-to-power plane shorts or any other types of plane-to-plane shorts, as opposed to linear trace applications. Typical linear trace applications include: tracing lines on a PCB; tracing a cable; open testing; and test and inspection of x/y lines on flat panel display applications such as LCD (liquid crystal displays). Practitioners in the art have in the past used pickup coils to locate such defects, mainly in the following applications: linear tracing of cables for shorts within the cable, linear tracing of lines on a PCB, and shorts between x,y lines on panel displays all while driving an oscillating current through the defect. The magnetic Faraday induction approach to be described in this invention is specifically directed towards plane-to-plane shorts and I/O nets-to-power plane shorts.
TSM metal patterns are extremely dense and expensive to make. In such instances, it may be simpler to detect a short with an ohmmeter if the pads on the BSM are provided with contacts to the power planes. Although detecting this type of defect may appear simple, locating the actual defect may be a formidable task since hundreds of thousands of places in the TSM may exist where two power planes or a power plane to an I/O net can short to each other. If two ground shielding planes are built parallel and in close proximity to one another, interlevel shorts can occur anywhere in the plane. It may be difficult and ineffective to rely on unassisted manual inspection of the entire area. It is, therefore, desirable to have an effective area scan method to locate this narrow class of yield detracting defects.
In U.S. patent application Ser. No. 08/807,076, plane-to-plane shorts are localized by an apparatus that includes: a current source; a means for applying current to the plane-to-plane short; an inductive magnetic field sensing probe for sensing changes in the magnetic field around the short; and circuitry for detecting a signal induced in the magnetic field sensing probe, when the probe is in the vicinity of the defect. This approach may subject the fragile electrical component to high current stresses and may lack adequate sensitivity for the more difficult case of finding high resistance power plane-to-power plane shorts.
Other related patents describe various electromagnetic probes to scan electronics packages in a variety of ways, although none address plane-to-plane shorts. The patents listed hereinafter can be divided into several groups according to their respective application: linear current trace probes with probes used as field sensors (electromagnetic); contactless techniques involving electrical testing using radiated electromagnetic signatures, wherein the probe is a field emitter and possibly also a sensor. None of these applications, however, address the special problems and concerns of power plane-to-power plane shorts or high resistance power plane-to-power shorts. Nor do they address the simplified case of I/O net-to-power plane shorts. Nor do they separate signals caused by noise or unwanted capacitive effects from the signal due to a power short.
Techniques for using electromagnetic sensor probes for scanning electronics packages are described in U.S. Pat. No. 5,406,209 to Johnson et al; U.S. Pat. No. 5,714,888 to Naujoks; U.S. Pat. No. 5,218,294 to Soiferman; U.S. Pat. No. 3,992,663 to Seddick; U.S. Pat. No. 4,186,338 to Fitchenbaum; U.S. Pat. No. 5,073,754 to Henley; and U.S. Pat. No. 4,542,333 to Koontz. All of the above utilize a magnetic field sensing probe as opposed to an electromagnetic field generating probe. These patents can be further classified into various groups highlighting different methods, as will be discussed below.
Johnson's U.S. Pat. No. 5,406,209; Naujoks' U.S. Pat. No. 5,714,888 and Soiferman's U.S. Pat. No. 5,218,294 describe an array of broad spectrum electromagnetic sensing probes to scan a module undergoing a functional test in a go/no-go test. In the case of a power plane-to-power plane short, an ohmmeter is presumed to have already detected the presence of a short. In all the methods described above, an electromagnetic signature which was previously generated, is compared to a predetermined signature. A decision is made whether the PCB is good or bad. Generally, all the radiated electromagnetic energies sensed with the predetermined signature of the profile are compared, although the electromagnetic sensed energy is not necessarily helpful in locating a power plane-to-power plane short. A signal which strongly pinpoints the position of a voltage plane short is generated by a magnetic field. In contradistinction, an electrostatic field tends to be a degrading component of the signal due to the presence of a noticeable capacitance between planes or between a plane and the probe. It is, therefore, not considered to be helpful, but rather a hinderance. None of the above patents make a distinction between meaningful and non-meaningful signals. Furthermore, none are sensitive to weak magnetic signals in combination with noise. Nor do they address capacitive effects caused by the plane-to-plane capacitance or by the probe-to-the board under test capacitance. A field sensing probe assumes that the module under test can withstand the necessary current stress to generate an adequate signal to create a field sufficiently large to locate the defect.
Seddick, in U.S. Pat. No. 3,992,663 uses a tape head mounted on an automatic positioning arm to trace a shorted net to locate the short. Similar to Johnson, Seddick uses a probe as a sensor, e.g., a magnetic pick-up device, and not as a magnetic field generating device. This is not as desirable with regard to sensitivity considerations. Furthermore, it is less non-destructive for reasons previously mentioned as the PBC under inspection is stressed by current to produce a signal for the scanning head. The PCB under test may be limited in the amount of current stress that it can be subjected to. This does not address power plane-to-power plane shorts which are not a linear net trace nor does it address high resistance shorts.
Fitchenbaum, in U.S. Pat. No. 4,186,338 describes a manual version of this using a tape head rather than a pickup coil. This approach is also a linear current trace application using a pickup coil probe as a sensor. It uses a simple threshold for detection and will become less effective as noise increases and as the signal decreases.
Both Seddick and Fitchenbaum make a simple comparison of electromagnetic signals received exceeding a given threshold to locate a path for the current. No attempt is made to distinguish meaningful magnetically caused signal from capacitively caused, electrostatically induced signal. Likewise, a broad spectrum is used that lacks the necessary sensitivity to a weak signal buried in noise. This method is also not extendable to high resistance shorts. Furthermore, a single net is being traced. This differs from the approach described herein, wherein a two-dimensional field containing power planes and/or thousands of nets is tested and inspected simultaneously, in which any I/O net is shorted to a power plane.
Henley, in U.S. Pat. No. 5,073,754, is a hybrid of the previously mentioned patents using an array of magnetic field sensors and driving the x,y lines of an LCD display. Searching for defects in x,y lines of LCD displays differs considerably from searching for power plane-to-power plane shorts. In the case of densely packed x,y addressing lines, many parallel x signal lines cross other parallel and distinguishable y lines with the possibility of a short occurring somewhere. In the case of power plane shorts, two individual large areas may be parallel to one another and short anywhere in the entire area. Henley does not address power plane shorts, using instead broad spectrum, current stresses, without distinguishing between meaningful magnetic signal and electrostatic degrading signal. This approach is limited to locating shorts in electronic packages having dense parallel x,y addressing lines, e.g., LCD, but it does not address locating power plane-to-power plane shorts.
Koontz U.S. Pat. No. 4,542,333 uses a Hall effect probe sensor to linearly trace lines in an automotive heating coil (of the type that may be used to heat the rear window of cars). The current must flow through the entire serpentine heating coil. This approach addresses looking for shorts or opens in a narrow application of simple automotive heating coils as a go no go test, but not locating power plane-to-power plane shorts in highly complex boards or modules.
Several other related patents use electrostatic and/or magnetic field sensing probes to scan electronic packages. These include: Khazam U.S. Pat. No. 5,486,753, Sheen U.S. Pat. No. 5,578,930, and Soiferman, both U.S. Pat. Nos. 5,551,110 and 5,424,633. They relate more to testing than to inspection techniques, namely, to determine the goodness of the device under test. These are not pertinent to locating power plane shorts and do not address the case of locating high resistance shorts between power planes which demands a much higher sensitivity. These are test approaches collecting an electromagnetic signature and making a direct compare with a predetermined signature for a go/no go decision. There is no need for such a complex go/no go test for power plane-to-power plane shorts since, as previously mentioned, a power plane short is easily detected with an ohmmeter. The need is not related to detection but rather to the location and inspection of plane-to-plane faults and these methods do not address that necessity.
Khazam, in U.S. Pat. No. 5,486,753 describes a method to induce a capacitive electrostatic signal into a populated PCB, connecting the individual output pins of ICs on the board to selected electronics in order to detect opens capacitively. This is an open test and not a short test, and consequently is not transferable to detect shorts between power planes.
Sheen, in U.S. Pat. No. 5,578,930 describes a method for inducing an electrostatic or magnetic signal into a populated printed circuit board and looking at the individual output pins of Ics on the board with appropriate electronics at the composite signal, and more specifically to whatever combination is usable as a signature. Likewise, Sheen uses a broad spectrum which is not sensitive to a weak signal buried in noise. This method is not extendible to high resistance shorts. Both Sheen and Khazam require much more complex hardware than is necessary for the narrower case of power plane-to-power plane shorts. Further, Sheen teaches an open test and a method of locating power plane-to-power plane shorts.
Soiferman, in U.S. Pat. No. 5,551,110 and 5,424,633 uses an induction coil to induce a voltage gradient in a PCB to test for functionality and quality. This approach also makes a simple comparison of electromagnetic signal received exceeding a threshold to locate a current path. No attempt is made to distinguish magnetically included signal from capacitively induced signal or from random noise. Soiferman looks at the signal as a composite for any effect as a go/no go test. Likewise, Soiferman uses a broad spectrum which is not adequately sensitive to a weak signal buried in noise. This method will not be extendible to high resistance shorts. As mentioned previously, there is no need for a such a complex go/no go test for power plane-to-power plane shorts which are easily detected by an ohmmeter. The need is for a method of locating these shorts for repair and diagnostic purposes in a cost effective manner.